Injection molding, and injection molding plastic parts in particular, has become a very cost effective method for quickly manufacturing parts. Typically, a single plastic resin is selected and prepared for injection into the mold. The result is a plastic part with uniform physical properties.
Sometimes a part may be formed from two different plastic resins. In this scenario, a first material is injected, then after a predetermined time period (usually determined by the amount of time needed for the melted resin to set), a second material is injected. A common term of art used to describe this method is the “two shot” method. In this method, the first material is shot into the mold. The mold can be repositioned and then can receive the second material. The two shot method can have associated disadvantages. First of all, there is an increased cycle time to produce molded part. That is, in order for a complete part to be produced, it takes one cycle of for the first material, and then one cycle for the second material. Secondly, there is often a perceptible line at the interface of the two different materials. Oftentimes, this visible line may be undesirable.
There can be some designs when it is desirous for a single part to have varying physical properties. For example, a designer may want a first section or region of a part to have a first amount of stiffness, and a second section of the part to have a second amount of stiffness. Single shot injection molding typically only provides uniform physical properties throughout the entire part. On the other hand, a two shot process can introduce a second material (providing a second physical property) but with added cycle time, and therefore, added cost. Further, the resulting product may no longer be visually acceptable due to visible interface lines between the first and second materials.
Therefore, what is desired is a cost effective way to produce an injection molded part with non-uniform material characteristics in a short process.